In this study we investigated an immunosorbent, E. coli blocking fluid (BF), proposed for use in the Legionella Indirect Fluorescent Antibody Test (IFA). With crossed immunoelectrophoresis (CIE) of clinically relevant Legionella species, only one heat-stable antigen (no. 1) cross-reacted with the BF preparation. Patients' sera with elevated Legionella IFA titres did not react with this antigen in CIE. Out of 23 IFA positive patients' sera, six had titres lowered significantly to negative, when BF was applied as serum diluent for the titration (IFA BF negative sera). All six sera were negative in the micro agglutination test (MA). None of the IFA BF negative sera contained any Legionella precipitins in CIE, whereas nine out of the remaining 17 IFA BF positive sera unchanged by BF contained one or more precipitins. CIE results could not explain the effect of BF in Legionella IFA, and further studies are needed to sufficiently define the use of immunosorbents in diagnostic Legionella serology.